1. Field of the Invention
This invention relates to optical encoders. More particularly, this invention relates to a so-called three grating type optical encoder including: three (in a case of a transmission type encoder) or two (in a case of a reflection type encoder) scales each formed with periodic gratings of three types: a light source for illuminating the gratings of the three types; and a light receiving element for detecting an illuminating light restricted by the gratings of the three types; wherein a periodic detection signal is produced in accordance with a relative displacement between two members.
2. Description of the Prior Art
In a machine tool and the like, as a device for measuring a relative displacement between a stationary member and a movable member, there are known displacement measuring devices including an optical encoder for producing a periodic detection signal in accordance with a relative displacement and a counter for turning the detection signals into pulses, counting and integrating the same.
As the optical encoder, in addition to a conventional encoder wherein changes in overlapping of gratings of two types is utilized, there is known a so-called three grating system, wherein changes in overlapping of gratings of three types 12, 14 and 16 are utilized, as shown in FIG. 11. The main principle of this three grating system is shown in Journal of the Optical Society of America, 1965, vol. 55 , No. 4, pp 373-381 for example, and disclosed in U.S. Pat. No. 3,812,352 and British Patent Application No. 44522/74, in imperfect forms though.
FIG. 11 is the system disclosed in Society of Photo-Optical Instrumentation Engineers (SPIE), Vol. 136, 1st European Congress on Optics Applied to Metrology (1977), pp 325-331.
To simplify, as shown in FIG. 11, this three grating system includes: a first grating 12 of a grating pitch P1; a second grating 14 of a grating pitch P2, which is provided at a position spaced a distance u apart from the first grating 12; a third grating 16 of a grating pitch P3, which is provided at a position spaced a distance v apart from the first grating 12 as opposed to the second grating 14; a light source 18 for emitting a diffusive illuminating light in directions of the first and third gratings 12 and 16 through the second grating 14; a light receiving element 20 provided behind the third grating 16, for detecting an illuminating light restricted by the first to third gratings 12, 14 and 16 and photoelectrically transducing the same; and a preamplifier 22 for amplifying a signal from the light receiving element 20 and turning the same into a detection signal a; and, when the first grating 12 is displaced in a direction x, the detection signal a periodically changes as a substantial sine wave.
Incidentally, the relationship between the pitches of the aforesaid parameters P1, P2, P3, u, v and detection signal a is divided into two including a geometric system and a diffractive system for the definition, as shown in Table 1. In Table 1, 1 is a natural number and .lambda. is an effective wave length of the illuminating light.
TABLE 1 ______________________________________ Geometric System Diffractive System ______________________________________ P1 P1 P1 P2 {(u + v) / v} P1 {(u + v) / v} (P1 / 2) P3 {(u + v) / u} P1 {(u + v) / u} (P1 / 2) u u u v .apprxeq. (lu P1.sup.2) / (.lambda.u - 1P1.sup.2) (l = Integer of l .gtoreq. 1) pitch of P1 P1 / 2 detection (optically divided signal into two) ______________________________________
With the conventional three grating system as described above, in the case of the geometric system for example, if a length of a dark portion of the first grating 12=a length of a light portion=10 .mu.m, the pitch P1=20 .mu.m and the grating gap u=v.apprxeq.5 mm for example, then it is known that the pitch P2 of the second grating 14 becomes EQU {(u+v)/v} P1 =40 .mu.m,
and the pitch P3 of the third grating 16 becomes EQU {(u+v)/u} P1=40 .mu.m.
Furthermore, in the case of the diffractive system, if a length of a dark portion of the first grating 12=a length of a light portion=20 .mu.m, the pitch P1=40 .mu.m and the grating gap u=v.apprxeq.5 mm for example, it is known that the pitch P2 of the second grating 14 becomes EQU {(u+v)/u} (P1/2)=40 .mu.m,
and the pitch P3 of the third grating 16 becomes EQU {(u+v)/u} (P1/2)=40 .mu.m,
However, with the conventional three grating system, as shown in FIG. 12, although a direct current component DC of the detection signal a is sufficiently provided, an alternate current component PP of the periodic signal is low, so that an SN ratio required in an electric circuit in the latter stage cannot be fully satisfied.
According to the experiments conducted by the inventor, on eleven samples by the conventional method, in each of which, with the aforesaid geometric system, the grating pitch P1=20 .mu.m, P2=40 .mu.m, P3=40 .mu.m, the grating gap u=v.apprxeq. 5 mm, and all of the gratings had the equal lengths of light portions and the dark portions, the SN ratios defined by the following equation were 12% at the lowest, 17% at the highest and 14.7% at an average, so that no satisfactory SN ratio was obtained. EQU SN ratio=(PP/DC).times.100% (1)
Furthermore, in the case of the conventional method, in which, with the aforesaid diffractive system, the grating pitch P1=40 .mu.m, P2='.mu.m, P3=40 .mu.m, the grating gap u =v.apprxeq.5 mm, and all of the gratings had the equal lengths of the light portions and the dark portions, it was made clear that data thus obtained were the substantially same as above, so that no satisfactory SN ratio was obtained.
Further, when a reflection type encoder is to be realized with the three grating system, if merely the first grating 12 is formed on a reflection type main scale, and the second grating 14 and the third grating 16 on the index scale are commonly used, then the grating pitch P2=P3, thereby presenting the disadvantage that P2 and P3 cannot be changed in pitch from each other.